The design of an RF transceiver requires that the transmitter section and receiver section be isolated from one another in some manner so that the sensitivity of the receiver is not unduly degraded by the relatively high power transmit signal. A multitude of transceiver designs that are successful in accomplishing this objective are known by practitioners in the art.
A straightforward solution to the problem of transmitter/receiver isolation is to turn off all elements of the transmitter chain whenever the transceiver is operating in the receive mode. By shutting down the transmitter chain, particularly including the transmit oscillator, internally generated energy in the receive band of the transceiver can be virtually eliminated. Such temporal isolation of the transmitter and receiver generally exacts a penalty in response time of the transmitter, and hence a switching delay between receive and transmit modes, since highly stable oscillators require a relatively long period of time to start up and achieve stability prior to transmission. A typical crystal oscillator has a start up time on the order of a few milliseconds. In many systems, such as transceivers for voice communications, switching delays of a few milliseconds are entirely tolerable.
Transceivers for data communications, however, cannot tolerate lengthy switching delays since this would significantly reduce the communications bandwidth. In such systems, the transmitter must remain in a standby condition during the receive mode. Unless the transmitter and receiver operate on different channels (an impractical solution for a communications network with multiple transceivers) the transmitter must be isolated from the receiver by extensive RF shielding. For example, circuits may be housed within elaborate compartmented metal enclosures to confine stray radiation. Transceivers that require large amounts of shielding tend to be heavy, bulky and expensive.
The need addressed by the present invention is for a lightweight, compact and inexpensive RF transceiver that can switch rapidly from the receive mode to the transmit mode and yet still achieve high levels of receiver sensitivity. Such a transceiver, which can be constructed on a single unshielded circuit board, has particular application in a data communications network of the type described in U.S. Pat. No. 4,918,690.